Nuclear magnetic resonance imaging systems make use of very large permanent magnet structures to provide their magnetic fields. Typically, permanent magnets are arranged in side-by-side arrangement to form a cavity to receive to the patient, in which cavity the magnetic field has a prescribed strength and orientation. Owing to the relatively large size of the cavity, the field produced by the permanent magnets is relatively low and it becomes necessary to utilize a great deal of magnetic material to obtain a useful field strength. The magnets therefore tend to be quite expensive. Significant reduction in the volume of magnetic material required could result in the substantial reduction of the entire nuclear magnetic resonance imaging system.
The usual manner for designing such permanent magnet cavities has been a very approximate one. It is the common practice for the designer of such a magnetic cavity to rely upon well-known structures so as to achieve the desired field or to modify such a structure somewhat by experimentation, in order to approximate the desire field more closely. In any event, the final magnetic structure only loosely approximates the desired result and makes inefficient use of the magnetized material in order to do so. This results in a substantially greater expense being incurred with respect to the permanent magnetic than is necessary.
Another inefficiency in the structure of permanent magnetic cavities arises from the need to confine the magnetic field within the permanent magnet structure. Typically, this has been achieved by providing a high permeability cover or "yoke" on the exterior of the permanent magnet structure, in order to provide a low reluctance return path for the magnetic flux. However, the yoke adds to the size, weight, and expense of the magnetic structure, as well as its cost. Furthermore, the structure utilizing a yoke does not make efficient use of the magnetic field available within the permanent magnet material and, therefore, requires the use of more material than is necessary.
It is therefore an object of the present invention to provide a yokeless permanent magnet cavity in which the magnetic field precisely matches a prescribed field in both magnitude and direction.
It is another object of the present invention to provide a method for constructing a yokeless permanent magnet structure which contains a cavity in which the orientation and magnitude of the magnetic field closely match predetermined values.
It is yet another object of the present invention to provide a method for constructing a yokeless permanent magnet structure having a cavity in which the magnetic field conforms closely with a predetermined orientation and magnitude, while permitting minimization of the amount of magnetic material used.
It is also an object of the present invention to provide a method for constructing a yokeless permanent magnet structure having a cavity in which the magnetic field has a prescribed magnitude and orientation, which method is readily amenable to application in automated equipment, such as computerized design systems.
In accordance with the present invention, the construction of a yokeless permanent magnet structure with a cavity is performed by first selecting a ratio "K" between the magnitudes of the prescribed field intensity within the cavity and the magnetization within the permanent magnet material. A reference point "F" for the construction is also selected within the cavity, such that point F lies on a equipotential line of zero potential. The point F must, furthermore, be selected so that it lies on a line l.sub.0h which divides the magnetic flux within the cavity into two separate loops or parts. Beginning from point F, the magnet structure is then constructed from a series of precisely abutted prisms of magnetic material which form a cavity with a predetermined, uniform polygonal cross section. Using well known boundary value conditions between the cavity and the various prisms of the magnetic material, as well as the restriction that the exterior surface of the permanent magnetic must be an equipotential surface of zero potential, each prism, in turn, is constructed, until the entire permanent magnet is completed. It has been found that the amount of magnetic material utilized in the entire permanent magnet structure obtained in this manner is uniquely related to the value K and the location of point F. It is therefore possible to quantify the relationship between the amount of magnetic material and the structure with respect to K and F, whereby a structure with the minimum amount of magnetic material may be obtained.